Overweight and obesity are risk factors for a range of health problems. The prevalence of overweight and obesity has been steadily rising among every segment of the U.S. population. In searching for factors driving the obesity epidemic, the overweight/obesity research is now focused on the balance between diet and physical activity (PA). To address the obesity epidemic, researchers have begun to explore the wider contextual determinants of obesity-related outcomes in hope of identifying new approaches to promoting PA and curbing the rising tide of obesity. This study attempts to contribute to the literature by examining the extent to which neighborhood built environments and socio-demographic contexts influence individual PA and weight status in the United States. To guide this investigation, we employ the social cognitive theory and ecological models and use multiple data sources including the 2000 Census data, GIS-based data from various sources, other place-based aggregate data, and continuous NHANES from 1999 to 2008. We ask: 1) How do contextual built environmental and socio-demographic factors affect objectively and subjectively measured PA and weight status over and above individual characteristics? 2) What are the interacting and mediating effects among contextual built environmental and socio-demographic factors in their impacts on PA and weight status? Are these observed contextual effects partly mediated by individual psychosocial pathways? 3) How do neighborhood factors interact with individual factors to impact PA and weight status? 4) How are the above relationships sensitive to geographic aggregation methods of local neighborhoods? 5) How does spatial dependency among local neighborhoods affect the observed patterns produced from the above analysis? This project goes beyond current research in five ways. First, it examines a wider range of place-based contextual features at both tract- and county-levels. Second, it is a national study covering the entire U.S. Third, it examines both self-reports and objective assessments of PA and weight status. Fourth, it provides a unique opportunity to examine interactions across a number of levels (e.g., individual- neighborhood, neighborhood-county). Fifth, it tests under-studied hypotheses in this literature such as the Modifiable Areal Unit Problem and the spatial dependency issues. This project combines perspectives from sociology, urban planning, geography, and spatial epidemiology;it bridges these disciplines to formulate research that addresses a set of questions vital to research on environmental determinants of PA and weight status. PUBLIC HEALTH RELEVANCE: This study examines how geographic and contextual factors at census tract level and county level are associated with individual-level physical activity and body weight in a national sample. Both the built environments and socio-demographic contexts will be considered. Given the rising tide of the obesity epidemic in the United States, research, strategies and programs for weight maintenance and weight reduction have become a high public health priority in the recent decade.